Draft: Feature/face tensor average

What this MR does / why does DuMux need it:

I had a look at the current implementation of faceTensorAverage and I think it makes more sense to use the projection matrix nn^T instead of identity I.

For the flux calculations it should not make any difference since T_avg_old \times n = T_avg_new \times n.

Notes for the reviewer

TODO: insert text here

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